only 5 wifi devices can be active at a time, after high data month

if 5 devices have wifi turned on and a sixth tries to get on our network it will not connect. we have to turn wifi off of a device  for it to work. I've tried two routers, did factory resets and made sure DHCP settings would allow 100 devices. I reset the modem and see that last month we went over on data, which is around when the problem started.